Conventional internal-combustion engines, both Otto and Diesel, rely principally on the in-line slider-crank mechanism. In this type of engine, a connecting rod couples each piston directly to a common crankshaft. Another basic type of engine configuration, that has been used in some diesel engines, is known as a swing beam engine. Swing beam engines date back to the turn of the century, and are shown schematically in FIGS. 1 and 2, labelled "Prior Art".
Referring to FIG. 1, a known swing beam internal-combustion engine includes a cylinder 10 and piston 12, with a swing beam 14 pin-connected to the piston 12 by a connecting link 16. The swing beam 14 is fixedly pivoted, at its opposite end, at pivot point 18. Finally, a connecting rod 20 pivotally connects the swing beam 14 and a crankshaft 22. The crankshaft 22, connecting rod 20, swing beam 14, and fixed pivot 18 constitute a four-bar rocker linkage of which the swing beam is the rocker.
Normally, two mirror-image swing beam configurations are connected back-to-back, as shown in FIG. 2, resulting in opposed pistons sharing a common cylinder. In FIG. 2, mirror image components are numbered with the designation "a". The crankshaft 22, containing crank arms 23 and 23a, actuates both halves of the mechanism. The balance of forces in this arrangement is favorable, except for the rotational inertia of the connecting rods 20, 20a, and some asymmetry in the motion of the connecting rods, which is imparted to the swing beams and pistons. Combustion takes place internally, in the space between the pistons 12 and 12a.
A number of variations of the basic arrangement shown in FIG. 2 have been proposed. For example, in an article entitled "Quiet Swing Beam Has Variable Compression Ratio", D. Scott, Automotive Enqineerinq, April 1977, pages 10-12, the author proposes mounting the lower ends of two swing beams, in a two stroke swing beam diesel engine, in eccentric bearing shafts. According to this proposal, a spring tensioned, toothed belt is arranged between pulleys on the two eccentric shafts and, when the engine is running, the torque pressure of the beams against the eccentrics moves the belt slightly against its retaining spring, thus shifting the fulcrums to reduce piston travel. This permits starting a diesel engine at a 20:1 compression ratio. When the engine is running, the compression ratio is lowered to about 13:1.
In the Scott engine, top dead center of the pistons occurs at bottom dead center of the crank. Because of this, crankpin motion is subtracted from the straightening movement of the connecting rod during the first part of the power stroke. This has the effect of slowing piston movement at the beginning of the power stroke (although the overall duration of the power stroke is the same as in conventional slider-crank engines), which improves combustion. The Scott article also suggests that an eccentric coupler can replace the piston connecting rod.
The piston connecting rods (i.e., the rod between the piston and swing beam) in a swing beam engine can be much shorter than the piston connecting rods in a conventional slider-crank engine, while maintaining the same angular swing of the connecting rod relative to the central piston axis. Possibly, the rods could even be replaced by eccentrics as suggested by Scott. However, it would be even more desirable to provide a swing beam engine configuration having a more compact design that does not require a connecting rod or eccentric between the piston and swing beam. It would also be desirable to provide a swing beam engine in which the piston displacement can be controlled during the engine cycle to produce an improved thermodynamic cycle, for example a longer power stroke relative to the compression stroke (as in Atkinson cycle operation), variable cycle operation and cycle timing, variable stroke and variable compression ratio.
Other internal-combustion engines have been proposed in an attempt to improve the performance of conventional slider-crank engines. These include the offset slider-crank engine, planetary drives and other engines based on standard, single degree of freedom mechanisms, the proportions of which have been altered to improve engine efficiency. In all such engines of which we are aware, the original engine construction or motion, which is symmetrical, is skewed (i.e., rendered unsymmetric) in order to produce the varied engine operating characteristic. By doing so, however, the resulting unsymmetrical mechanism possesses such unfavorable dynamic unbalance and force-transmission characteristics that the engine is impractical.